Evaluating flow through 2" pipe from high to low pressure vessel

[GeorgeGrande]

I am trying to find a way to evaluate the N-Pentane flow through a 2” pipe leading from the bottom of a pentane vaporizer (saturated liquid area) operating at 21 barg to a low pressure vessel designed for up to 8 barg (nominal pressure is atmospheric).

The 2” pipe equivalent length is about 40 m with no elevation change.

 

[BigInch]

What do you mean by "evaluate"?

I'd think you'd want to have a control valve, and/or an orifice plate AND an ESD valve in that line.



**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[GeorgeGrande]

BigInch, thanks for your quick reply. Since the low pressure vessel is used for maintenance only I prefer not using control valves (the spoken scenario is a mistake and not a operational procedure).

My aim is to define the pressure relief valve on the top of the low pressure vessel (see sketch attached to my first mail).
I checked fire size but than I came to realize that this 2” valve, that might be mistakenly open, can cause the worst case scenario. So, if the flow in this line is known than I can compare between both cases and specify the PSV.

 

[BigInch]

A lot of accidents happen during maintenance activities.

Personally I think I might have an orifice plate to take cut some pressure going to a 2" regulator, then an ESD valve downstream of that, then a relief valve downstream of that, all on the 2" line, then a big PSV on the LP vessel,
.... maybe two,

but I believe in being as safe as possible.

These are just 2" valves. Are they too expensive?

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[GeorgeGrande]

Dear Biginch,

1. I agree that most accidents happen during maintenance and that is why I am looking for your wise advice. This system already exists and valve cannot be added (maybe on the future design). I need to deal with this line as is.
2. An orifice plate is not acceptable since this is a two way system that is being used also in the reverse way. Operators object to an orifice since it will slow up the filling procedure (from LP to HP).
3. ESD and PSV on the 2” line will not solve my safety problem. I checked it before since I thought it will be cheaper solution.
4. I don’t want to get in to the all system design since it will be hard to explain over these pages, but believe me that I took the problematic part of this system and isolated it in order to start this thread.
5. The system is designed so the 2” line will be diverted directly into the LP vessel via 40m 2” line. Initially I calculated the flow as a flashing fluid through a 2” orifice ignoring the pressure drop on the line. I think that was a mistake.

So now I am looking for a way to evaluate the flow through this line taking in consideration the DP and friction loss. Are we talking about two phase flow? Full flashed vapor? Choked flow? And how can I quantify it?

 

[BigInch]

Looks like 2 phase possible, which will flash at the block valve, or later in the line whenever it reaches vapor pressure.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[katmar]

Your first post said the n-Pentane was saturated at 21 barg so there will be progressive flashing all the way along the pipe. This makes it a 2-Phase flow problem.

You need to do an energy balance to determine whether it is fully flashed. Guessing some numbers makes it look like about 40 to 50% will flash. The amount of flash will be important in your specification of the PSV on the LP vessel. The PSV will initially see only the flashed portion as vapor, but if the vessel fills completely it will see a 2-Phase mixture.

You need to do the detailed calculations to determine whether it is choked flow or not. With such a high degree of flash it would not surprise me if it is choked.

How to quantify it? What tools do you have for 2-Phase flow calcs. Unless you have some fairly sophisticated software this looks like a rather lengthy trial and error calculation.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[GeorgeGrande]

Thanks.

My “thumb” guess was that I am dealing with flashed two phase flow that will finally be vapor (in line or tank).
I made a T&E simulation taking a 21 barg saturated liquid going into a 40 m 2” line (CS) and change the flow until I got 8 barg in the end of the line. Larger flow will drop the pressure down. I used some friction loss correlation in my simulation program (dukler,darcy).
I presume that the flow I have got will be the maximum. Vapor fraction in all cases is ~0.6 at pipe discharge.

What do you say about this logic?

 

[katmar]

Yes, that is the logic I intended - i.e. use trial and error varying the flow until the calculated pressure drop matches the actual. Which simulator are you using?

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[GeorgeGrande]

I used winsim (design II) for this. There is no need for more complicated tools. Do you have any recommendation regarding the friction factor and friction loss correlation?